This invention relates to the molding of shaped polymeric articles especially contact lenses. This invention more particularly relates to the molding of shaped polymeric materials from silicone-containing monomeric mixtures.
Various molding processes are known for the production of contact lenses. Among those processes are spincasting, static cast molding, and related processes. Spincasting methods are disclosed in U.S. Pat. Nos. 3,408,429 and 3,496,254. Examples of static cast molding processes are found in U.S. Pat. Nos. 4,113,224 and 4,197,266. An illustration of a related molding process is U.S. Pat. No. 4,555,372, which teaches a process comprising spincasting followed by lathe cutting.
Conventional soft contact lenses are prepared from monomeric mixtures consisting predominantly of hydrophilic monomers such as 2-hydroxyethyl methacrylate, N-vinylpyrrolidone, and others. Oxygen permeability of such conventional hydrogel materials is a result of the water content of the materials. Recently, attempts have been made to increase the oxygen permeability of soft contact lenses by the use of silicone-containing monomers. Silicone compounds are usually very hydrophobic, diminishing the wettability of silicone-containing soft lenses.
Another class of soft contact lenses--nonhydrogel soft contact lenses--are known as silicone elastomer lenses and are prepared from silicone rubbers. These lenses are highly permeable to oxygen and can easily transmit sufficient oxygen to maintain the normal respiratory process on the surface of the cornea. Unfortunately, silicone rubber is strongly hydrophobic and readily absorbs lipophilic substances which exist in the tear film. These properties result in discomfort to the lens wearer and have significantly diminished the market for this type of lens. Hydrophilic monomers have been added to silicone elastomer formulations to overcome these limitations but such attempts have not yet proven to be entirely satisfactory.
The class of contact lenses known as rigid, gas-permeable materials are also typically silicone-based copolymers prepared from siloxanylalkyl esters of methacrylic acid and other acrylate, methacrylate, or itaconate monomers in many known formulations. Such compositions, like the soft contact lens materials referenced above, exhibit enhanced oxygen permeability, due in large part to the use of silicone material. It is desirable to increase the silicone content to further enhance oxygen permeability. However, increasing silicone content diminishes wettability of the material.